I removed my center section of my floor on my baja 24 outlaw. It's in really good shape except for the edges in a couple of spots. I figured since I have it out and I'm replacing the carpet, I might as well replace this section. I'm not going to replace the outer sections since they are in good shape with no flex. I have 2 questions:
1) Do I need to use marine grade plywood or just regular plywood? I'm planning on sealing it with resin and mat on both sides.
2) Anyone known what thickness I should use? It seems like the current piece is between 5/8 and 3/4 thick. I don't want the center section to protrude up to much making the rear seat hard to install since I'm not replacing the outer sections. I could always add another layer of mat to the outer sections to even out I guess.

Looks like a good start. Get a good quality exterior-grade plywood with a high ply count and decent face plys. Go to a true lumber yard and not the big box home improvement shops. They will either already have what you need or can readily get it. Depending on the span, you might add some ribs to the bottom side made from extra plywood pieces and glass it all together. Marine-grade is not necessary since you plan on sealing both sides anyway. The high ply count is what counts! Sand and use a quality urethane and/or epoxy paint. Sherwin Williams has a good selection.